Predominantly aliphatic lower hydrocarbon materials from carbonaceous solids

ABSTRACT

Predominantly aliphatic lower hydrocarbon materials may be obtained from carbonaceous solids by a sequence of steps comprising gasifying the carbonaceous solids, combining the gasification product stream with first and second internal recycle streams, separating certain impurities from the combined stream followed by cooling to liquefy and separate the hydrocarbon materials, directly reacting the remaining carbon monoxide and hydrogen in the presence of a catalyst to produce a first internal recycle stream, fractionating the separated hydrocarbon materials to separate and recover the predominantly aliphatic lower hydrocarbon materials from the higher hydrocarbon materials, subjecting the higher hydrocarbon materials to steam cracking to produce a second internal recycle stream, and recycling the internal recycle streams for combination with the gasification product stream as noted above.

This invention relates to an integrated process for producingpredominantly aliphatic lower hydrocarbon materials from carbonaceoussolids.

In the past, hydrocarbons of a predominantly aliphatic nature, bothparaffinic and olefinic, have generally been obtained from distillationand separation of crude petroleum and natural gas and from crackingpetroleum fractions. Quite apparently, with the decline of availablereserves of crude petroleum and natural gas, there is a need foralternative means for producing such predominantly aliphatichydrocarbons which are not dependent upon natural gas and petroleumfeedstocks.

Another source of hydrocarbons has been from distillation ofcarbonaceous solids, such as coal. However, these hydrocarbons arehighly aromatic and constitute only a minor percentage of the initialcharge of solids.

The Synthol process is also known (Chem. Engr. Prog., 1960, Vol. 56, No.4, pp 39-48) for producing hydrocarbons from coal, some of which arealiphatic. In this process, coal is initially gasified, then purifiedand used as a feedstock to both an Arge synthesis and a Kelloggsynthesis. The purified effluent may be used directly as a feedstock toan Arge synthesis for producing primarily diesel oil and waxes. However,in the Kellogg synthesis wherein gasoline is primarily produced, thepurified effluent is first passed through a reforming section.Consequently, while the process may be operated to produce somealiphatic hydrocarbons, it is necessarily relatively complicated.

In accordance with this invention, there is provided an integratedprocess wherein lower hydrocarbons and some oxygenated lowerhydrocarbons of a predominantly aliphatic nature are produced fromcarbonaceous solids by a combination of gasification and Fischer-Tropschsynthesis without reformation. Briefly described, the integrated processinvolves gasification of a carbonaceous solid under suitable conditionsto produce a gasification product stream of which the major portion isformed by carbon monoxide, carbon dioxide, hydrogen, and water, with aminor portion being hydrocarbons of a predominantly aliphatic nature.Since most carbonaceous solids contain sulfur materials, some sulfurcompounds, such as hydrogen sulfide and carbonyl sulfide, will also bepresent. The gasification product stream is then combined with twointernal recycle streams rich in hydrocarbon materials of apredominantly aliphatic nature, and the combined stream is subjected toconventional separation treatments for removal of most sulfur compounds,carbon dioxide, and water. The partially purified stream is thensubjected to cryogenic separation to separate the predominantlyaliphatic hydrocarbon materials from the carbon monoxide and hydrogen.The latter are then used as a feedstock to a Fischer-Tropsch synthesisand are converted to a product stream rich in hydrocarbon materials of apredominantly aliphatic nature which becomes one of the internal recyclestreams referred to above. These hydrocarbon materials, paraffinic andolefinic, include both hydrocarbons and some oxygenated hydrocarbons.The predominantly aliphatic hydrocarbon materials separatedcryogenically are fractionated to separate the lower hydrocarbonmaterials from the higher hydrocarbon materials. Obviously, thisseparation may involve several fractions as desired. The lowerhydrocarbon materials are recovered as the predominantly aliphaticproduct, while the higher hydrocarbon materials are subjected to steamcracking to produce the other internal recycle stream.

The invention is applicable to carbonaceous solids in general, such ascoke, oil shale, tar sands, char, lignite, anthracite, and bituminouscoals. According to the invention, these carbonaceous solids aregasified by reaction with steam as is known in the art. In general,gasification is accomplished by feeding the carbonaceous solids,preferably in a particulate form, together with steam to a gasificationzone operating at temperatures in the range of 500° C to 2,100° C,preferably 550° C to 1,300° C, and pressures of about 1 atmosphere to300 atmospheres, preferably 1 atmosphere to 100 atmospheres. The ratioof steam (water) to carbonaceous solids is generally in the range ofabout 0.1 to 3 pounds of steam per pound of carbonaceous solids,preferably 0.5 to 2 pounds of steam on the same basis. Higher quantitiesof water will tend to excessively increase the amount of carbon dioxidein the product stream at the expense of carbon monoxide, the carbonmonoxide being desirable for the subsequent Fischer-Tropsch synthesis.Lower quantities of water tend to result in insufficient conversion ofthe carbonaceous solids for an efficient process.

The temperatures in the gasification zone may be obtained by anysuitable external heating means, such as gas heaters or electricalresistance means. Alternatively, and as is usually the case, thetemperatures may be generated in situ by introducing anoxygen-containing gas along with the carbonaceous solids and steam tothe gasification zone. This will result in some combustion taking placein the zone and generating the necessary thermal energy to sustain thedesired temperatures. It is pointed out that the combustion region inthe gasification zone may operate at temperatures somewhat above thosementioned heretofore. Some water will also be produced by the combustionand can be taken into consideration in adjusting the quantity of outsidesteam fed to the zone.

The oxygen-containing gas can be provided by any suitable source and cancontain some inert materials. Quite obviously, the inerts should bemaintained as low as possible so as to avoid the necessity of handlinglarge volumes of materials which do not contribute to the efficiency ofthe process. Preferably, an oxygen-rich stream containing at least 95mol percent oxygen is used, such as that provided by a Linde-Franklprocess. The amount of oxygen supplied to the gasification zone maygenerally range from about 0.1 to 1 pound of oxygen per pound ofcarbonaceous solids, preferably in the range of 0.2 to 0.8 pound ofoxygen on the same basis.

The initial effluent from the gasifier may contain tars and relatedheavy materials such as result from volatilization of coal. Typical ofthese materials are highly aromatic compounds, phenolics, etc. Usually,when tars and the like are present, a quenching stage, such as withwater, is employed to condense them from the effluent, and they are thenrecycled back to the gasifier. Not all gasifications of carbonaceousmaterials result in tars being present in the initial effluent, and thusquenching is not always necessary. Quenching may also serve to removeparticulate solids carry-over or, alternatively, a conventional solidsseparator may be used if it is desired or necessary to remove suchsolids for recycle or other disposal. All of this is well known in theart.

In any event, a gasification product stream is obtained from thegasification step in the overall process. The gasification productstream is predominantly carbon monoxide, carbon dioxide, hydrogen, andwater, although some hydrocarbons of a predominantly aliphatic natureare present along with some tars. With sulfur-containing carbonaceoussolids, some sulfur compounds, such as hydrogen sulfide and carbonylsulfide, will be formed.

The gasification product stream is combined with the internal recyclestreams which are rich in hydrocarbon materials of a predominantlyaliphatic nature. These recycle streams are obtained as the effluentfrom a Fischer-Tropsch synthesis and as the effluent from a conventionalsteam cracking step further downstream and to be described in furtherdetail hereinafter. As noted above, the predominantly aliphatichydrocarbon materials, paraffinic and olefinic, include bothhydrocarbons and some oxygenated hydrocarbons. In addition to thehydrocarbon materials, the recycle stream also contains carbon monoxide,carbon dioxide, water, and hydrogen.

The combined streams, hereinafter referred to as the crude productstream, are then subjected to separation to remove carbon dioxide,sulfur compounds, and water. This may be accomplished according to knowntechniques. For example, water may be removed by simply cooling thecrude product steam to condense the major portion of the water followedby passing the crude product stream through a dryer containing adesiccant, such as alumina or silica gel. Carbon dioxide and some sulfurcompounds may be removed by such conventional techniques as adsorptionwith a suitable agent; e.g., an aqueous solution of an amine,particularly monoethanolamine, or by treating with a hot aqueoussolution of potassium carbonate, or by a combination of such processes(see U.S. Pat. No. 3,684,689 for applicable techniques). Alternatively,carbon dioxide and sulfur compounds, such as hydrogen sulfide andcarbonyl sulfide, may be removed simultaneously by means of the Rectisoland Purisol systems as described in Industrial and EngineeringChemistry, Volume 62, No. 7, July, 1970, pp 39-43. If desired, sulfurmay be recovered from the separation liquids by the well-known Clausprocess.

The thus purified product stream, containing essentially onlyhydrocarbon materials, carbon monoxide and hydrogen, is then subjectedto cryogenic separation. In such cryogenic separation, the purifiedproduct stream is cooled to a temperature below the condensationtemperature of methane, the lowest boiling hydrocarbon material presentin the product stream, thus effecting condensation of the hydrocarbonmaterials and separation from the carbon monoxide and hydrogen.Cryogenic systems for performing this separation are known in the art.The separated hydrocarbon materials are then subjected to furtherseparation and treatment as described hereinafter.

The hydrogen and carbon monoxide remaining after separation of thehydrocarbon materials are used as a direct feedstock for theFischer-Tropsch synthesis step of the process. This synthesis is wellknown in the art and is generally described hereinbelow.

The feedstock to the Fischer-Tropsch synthesis should have a mol ratioof hydrogen to carbon monoxide of at least 1/1, and preferably at least1.5/1. In most instances, the hydrogen and carbon monoxide from thecryogenic separator will be at a mol ratio much higher than this, but ifat any time the ratio falls below the 1/1 level, additional hydrogenfrom a suitable outside source should be added to bring the ratio to theproper minimum level.

Low amounts of hydrogen decrease the reaction rate and, perhaps moreimportantly, tend to result in some disassociation of the carbonmonoxide to carbon dioxide and elemental carbon. The formation of carbonis undesirable as it deposits out on the reactor walls and catalyst.This results in decreased heat transfer, a factor which may besignificant in view of the exothermic nature of the reaction, and adecreased activity of the catalyst. As the mol ratio of hydrogen tocarbon monoxide increases, the rate of reaction generally increases upto a point after which it either remains somewhat constant or eventapers off. In addition, high amounts of hydrogen also generally tend toresult in lower average molecular weight hydrocarbon materials beingformed with paraffinic compounds favored over olefinic compounds.Another factor to be considered with high amounts of hydrogen is thatthe unconsumed hydrogen must be carried through the recycle steps of theprocess since the effluent of the Fischer-Tropsch synthesis is recycled.Considering all of these aspects, it is generally desired to operate theprocess with a mol ratio of hydrogen to carbon monoxide of not greaterthan about 5/1, and preferably in the range of about 2/1 to 4/1.Obviously, a suitable outside source may be employed to introduceadditional carbon monoxide or hydrogen to provide the desired mol ratio.

It is pointed out that the hydrogen-carbon monoxide feedstock from thecryogenic separator may contain some hydrocarbon materials and may evencontain some water without adversely affecting the Fischer-Tropschsynthesis step. Sulfur compounds, such as hydrogen sulfide and carbonylsulfide, are undesirable as they tend to deactivate the synthesiscatalyst. These, however, are removed or reduced to a sufficiently lowlevel with the earlier purification steps.

The Fischer-Tropsch synthesis may be conducted at temperatures in therange of about 150° C to about 450° C. Lower temperatures tend to resultin higher molecular weight products which may cause fouling of thecatalyst or reaction zone. On the other hand, higher temperatures maytend to result in carbonization which likewise may cause catalystfouling. Preferred temperatures are in the range of 200° C to 400° C,with the most preferred temperatures ranging from 250° C to 350° C.

Pressures as low as atmospheric pressure may be employed, but thereaction rate is relatively slow at low pressures. Higher pressures mayalso be used with the primary considerations being equipment design,possible reactor and catalyst fouling due to the fact that higherpressures tend to result in higher molecular weight products, andreaction control since increased pressure increases the reaction rate.Generally, pressures in the range of 5 to 75 atmospheres gauge will beused, preferably 10 to 30 atmospheres gauge.

The reaction may be conducted in a zone containing a conventional fixedor fluidized (fixed or entrained types) catalyst bed. Normally, thefluidized bed is employed. Space velocities in the range of about 500 to50,000 volumes of feedstock/volume of catalyst/hour at standardtemperature and pressure conditions may be used, preferably in the rangeof 5,000 to 10,000 V/V/hr STP.

The catalysts useful for the reaction include any Fischer-Tropschcatalyst containing iron, cobalt, nickel, or ruthenium. These catalystsare well known in the art, being described in the Fischer-Tropsch andRelated Syntheses by Storch et al, John Wiley and Sons, 1951, Chapter 3,all of which is incorporated herein by reference. As also well known inthe art and described in the referenced text, these catalysts may besupported and may be promoted or activated by numerous materials. All ofthis is intended to be encompassed by the phrase "Fischer-Tropschcatalyst" as used herein. Further examples of suitable Fischer-Tropschcatalysts appear in U.S. Pat. No. 2,543,327 and U.S. Pat. No. 2,944,988,also incorporated herein by reference.

A particularly preferred Fischer-Tropsch catalyst is one containing ironand promoted or activated with alumina, magnesium oxide, calcium oxide,potassium oxide, silica, manganese oxide, thoria, titania, molybdenumoxide, or mixtures thereof. Suitable sources of iron component includemill scale and magnetite with the latter already containing some of thepromoters or activators.

The synthesis is preferably carried out under conditions within theranges described above to a conversion of carbon monoxide in thefeedstock of at least 50 percent. The effluent from the Fischer-Tropschsynthesis contains hydrocarbon materials which include both hydrocarbonsand some oxygenated hydrocarbons and are predominantly aliphatic. Byaliphatic, it is meant paraffinic and olefinic. Very little aromaticcontent is present. The effluent will, of course, additionally containcarbon monoxide, hydrogen, carbon dioxide, and water. The total effluentis then recycled to the process and combined with the gasificationproduct stream prior to processing through the separation zone.

The predominantly aliphatic hydrocarbon materials which are separated inthe cryogenic separation step are fractionally distilled to separate thedesired molecular weight products. For example, methane may be initiallyseparated as it is the lowest boiling followed sequentially by ethylene,ethane, propylene, propane, etc, as desired. Product streams ofprimarily a single component or a mixture of components may bewithdrawn. The remaining hydrocarbon materials not desired as products,usually the higher hydrocarbon materials, such as the C₃, C₄, or C₅ andhigher materials, are then passed to a conventional steam-cracking stagefor cracking to lower aliphatic hydrocarbon materials and subsequentrecycle back to the process to be combined with the gasification productstream prior to processing through the cryogenic separation zone.

The cracking step is accomplished with steam and high temperatures as isconventionally known in the art. In general, the cracking may be carriedout with about 0.2 to 1.5 pounds of steam per pound of hydrocarbonmaterials at temperatures ranging from about 750° C to 950° C andpartial pressures of the hydrocarbon materials of up to 2 atmospheres.If the hydrocarbon materials being cracked contain only C₃ and highermaterials, it is preferred that the hydrocarbon partial pressureemployed be up to only 1 atmosphere. Residence times in the range of0.01 to 10 seconds, preferably 0.05 to 1 second, may be satisfactorilyemployed.

As mentioned hereinbefore, the effluent from the cracking step is thenrecycled to the process and combined with the gasification productstream along with the Fischer-Tropsch effluent, and the combined streamis subjected to the separation processing described hereinbefore.

The following example will serve to further illustrate the process ofthe invention:

Example

A small charge of coal is placed in a cylindrical stirred reactor andcombustion of this charge is begun with oxygen and suitabletemperatures. After starting combustion of the initial charge of coal,coal gasification is conducted by feeding 33.5 pounds of pulverized coalper hour into the top of the reactor while simultaneously feeding oxygen(308 SCF/hour) and steam (55 pounds/hour) into the bottom of thereactor. As the coal, oxygen, and steam supply rates are brought up tothese values, the pressure of the reactor is also allowed to graduallyincrease to an operating pressure of about 350 psig (about 25atmospheres). After several hours of steady operation, the temperatureprofile of the reactor becomes approximately steady (decreasing fromabout 1,700° to 2,100° C at the bottom of the reactor to about 500° to600° C at the top of the reactor) and the gasification product stream(after quenching and removing tars) consists of a gas streamapproximately 2,000 SCF per hour and having the composition of 35.9percent steam, 17.8 percent CO₂, 15.3 percent CO, 24.2 percent H₂, 6.4percent CH₄, 0.2 percent H₂ S, 0.3 percent C₂ H₆, and 0.2 percent C₂ H₄(all by volume).

According to the invention, it is contemplated that this gasificationproduct stream is mixed with the product gas from the Fischer-Tropschreactor (see below) and with the products of the cracking furnaces (seebelow). The combined stream is cooled to about 25° to 75° C under about325 psig pressure to condense and separate water. The gas stream isconducted into a countercurrent absorption column containingethanol-amine and water wherein CO₂ and H₂ S are selectively removed.The clean gas, under approximately 325 psig pressure, is cooled througha series of heat exchangers in the cryogenic separator to temperaturesin the range of -100° C to -160° C where most of the methane and higherhydrocarbons present condense to liquids and are separated fromhydrogen, carbon monoxide, and a small fraction of methane which is notcondensed but remains gaseous. The liquefied hydrocarbons are thenfractionally distilled to a methane cut, an ethylene cut (contains asmall amount of acetylene), an ethane cut, and a propylene cut, whilepropane and higher boiling materials are left for introduction to thecracking furnaces.

The invention contemplates conducting the gaseous product from thecryogenic separator to the Fischer-Tropsch reactor where hydrogen andcarbon monoxide are converted to hydrocarbons. A simulated gaseousproduct formed of H and CO in a molar ratio of about 3/1 is conductedinto the reactor containing a fluidized catalyst prepared byhydrogenating and carbiding a pulverized fused mixture of magnetite,potassium carbonate, and alumina. The Fischer-Tropsch reaction ismaintained at a temperature of about 388° C, pressures of about 300psig, and with a space velocity of about 8,000 V/V/hour STP. The productfrom this reaction, after passing through a cyclone to remove entrainedcatalyst, is compressed to about 350 psig and recycled back to mix withthe gas from the gasifier and the product of the cracking furnacesaccording to the invention and as noted above. The product from theFischer-Tropsch reaction has the following approximate composition: 21.7percent H₂, 0.8 percent CO, 27.1 percent CH₄, 8.6 percent ethylene andethane, 7.3 percent propylene and propane, 4.0 percent C₄ hydrocarbonmaterials, 2.4 percent C₅ hydrocarbon materials, 5.0 percent C₆₊hydrocarbon materials, 4.8 percent CO₂, and 18.3 percent H₂ O.

The ethane cut and the propane and higher boiling hydrocarbons from thecryogenic separator are introduced to cracking furnaces operated undertwo sets of conditions to optimize the yield of products. In the ethanecracking furnace, the ethane cut along with 0.2 moles of water per moleof ethane and 50 ppm H₂ S is cracked in a tubular furnace at about 890°C coil outlet temperature, 22 psia coil outlet pressure, and an averagecontact time of about 0.5 second. The product from this crackingreaction contains about 20 mole percent water (steam), 26 percent H₂, 4percent methane, 0.2 percent acetylene, 26 percent ethylene, 23 percentethane, 0.4 percent propylene, and 0.8 percent higher boiling products.The propane and higher boiling hydrocarbons from the cryogenicseparation are cracked in the other cracking furnace, along with about0.6 mole of water per mole of hydrocarbon feed and 50 ppm H₂ S, at about775 to 800° C coil outlet temperature, 10 psia outlet partial pressureof hydrocarbons, and a contact time of about 1.3 seconds. The productfrom this cracking furnace contains about 38 mole percent water, 8percent hydrogen, 21 percent methane, 0.3 percent acetylene, 15.3percent ethylene, 4 percent ethane, 7 percent propylene, and 6.4 percenthigher boiling components. The product streams from both the ethanecracking furnace and the propane and higher boiling hydrocabon crackingfurnace are quenched with water to a temperature of less than 500° C andare then combined with the gasification product stream and theFischer-Tropsch reactor effluent as described hereinbefore.

Thus, the products from the overall process depicted by this example arethe methane cut, the ethylene cut, and the propylene cut from thecryogenic separator.

Thus, having described the invention in detail, it will be understood bythose skilled in the art that certain variations and modifications maybe made within the spirit and scope of the invention as described hereinand defined in the appended claims.

We claim:
 1. A process comprising:(a) gasifying carbonaceous solids withfrom about 0.1 to about 3 pounds of steam per pound of said solids attemperatures in the range of 500° C to 2,100° C and pressures ofatmospheric to 1,000 atmospheres to form a gasification product streamcontaining a mixture of water, carbon dioxide, carbon monoxide,hydrogen, sulfur compounds and hydrocarbon materials of a predominantlyaliphatic type, (b) combining the gasification product stream with firstand second internal recycle streams rich in hydrocarbon materials of apredominantly aliphatic type to form a crude product stream, (c)removing water, carbon dioxide and sulfur compounds from the crudeproduct stream to form a purified product stream containing essentiallyonly hydrocarbon materials, carbon monoxide and hydrogen, (d) coolingthe purified product stream to liquefy and separate the hydrocarbonmaterials from the carbon monoxide and hydrogen, (e) directlyintroducing the carbon monoxide and hydrogen from step (d) to acatalytic reaction zone and reacting the carbon monoxide and hydrogen insaid zone to produce a first internal recycle stream rich in hydrocarbonmaterials of a predominantly aliphatic type, said reaction beingconducted under conditions of temperatures in the range of about 150° Cto 450° C, pressures in the range of atmospheric to 75 atmospheres andspace velocities in the range of 500 to 50,000 V/V/hr STP, in thepresence of a Fischer-Tropsch catalyst containing iron, cobalt, nickelor ruthenium, (f) recycling the first internal recycle stream of step(e) back to step (b) of the process, (g) fractionally separating thehydrocarbon materials of step (d) into product hydrocarbon materials andresidual higher hydrocarbon materials, (h) directly introducing theresidual hydrocarbon materials to a cracking zone and cracking saidhydrocarbon materials to produce a second internal recycle stream, saidcracking being carried out using about 0.2 to 1.5 pounds of steam perpound of hydrocarbon materials, temperatures of 750° C to 950° C,hydrocarbon materials partial pressures up to about 2 atmospheres andresidence times of 0.01 to 10 seconds, and (i) recycling the secondinternal recycle stream of step (h) back to step (b) of the process. 2.A process according to claim 1 wherein the carbonaceous solids are coke,oil shale, tar sands, char, lignite, or coal.
 3. A process according toclaim 2 wherein 0.1 to 1 pound of oxygen per pound of carbonaceoussolids is additionally employed in the gasification of step (a).
 4. Aprocess according to claim 3 when temperatures in the range of 550° C to1,300° C, pressures in the range of atmospheric to 100 atmospheres and0.5 to 2 pounds of steam per pound of carbonaceous solids are employedin the gasification of step (a).
 5. A process according to claim 4wherein 0.2 to 0.8 pound of oxygen per pound of carbonaceous solids isadditionally employed in the gasification of step (a).
 6. A processaccording to claim 3 wherein the reaction of step (e) is conducted attemperatures of 200° C to 400° C, pressures of 5 to 75 atmospheres gaugeand space velocities of 5,000 to 10,000 V/V/hr STP.
 7. A processaccording to claim 6 wherein the cracking of step (h) is carried out ahydrocarbon materials partial pressures of up to 1 atmosphere andresidence times of 0.05 to 1 second.